Möller, Carla; Mutterlose, Jörg

Abstract

In the Early Cretaceous an epicontinental sea covered most of what is nowadays Northwest Germany. Sediments accumulated in a synsedimentary subsiding basin, the Lower Saxony Basin (= LSB), which formed the southern extension of the proto-North Sea. Two sedimentary successions of mid-Hauterivian
age (calcareous nannofossil zones BC8–9) were correlated by means of coccolith biostratigraphy. The sections represent (a) a marginal marine environment, <10 km distance from the former coastline, dominated by sandy deposits (Emlichheim), and (b) the basin centre, about 50 km offshore,
with clay dominated sedimentation (Resse). Our study aims at reconstructing the palaeoceanography of the LSB in the mid Hauterivian and at comparing the nannofossil assemblages of the two different settings. In the Emlichheim area (German-Dutch borderland) the Gildehaus Sandstone is represented
by coarse siliciclastics and intercalated mudstone intervals. The mud-dominated matrix contains rich and diverse calcareous nannofossil assemblages, which in combination with lithology allow for a sequence stratigraphic interpretation of the Gildehaus Sandstone. The mudstones of the Resse
section (basin centre) contain a rich nannoflora and an abundant ammonite fauna. The latter shows an alternation of endemic, Tethyan and Boreal taxa, which shed light on the palaeoceanographic conditions of the LSB in the mid-Hauterivian. Quantitative analyses of the calcareous nannofossil
assemblages show an offset between the abundances of the fertility indicators and In Emlichheim, is present in higher abundances than , in Resse is dominating. These results confirm
previous ideas regarding the palaeoecological preferences of the two species. At the same time they indicate palaeoceanographic differences between the both sections. In Resse, calcareous nannofossils show a pronounced increase in abundance that coincides with the diversification of the
ammonite fauna. The combined nannofossil-, ammonite and δO-datasets suggest, that the changes in the biota of the mid-Hauterivian (calcareous nannofossil zone BC8) are related to an increase in surface water fertility that goes along with fluctuations of relative
sea level and a moderate warming.